We now come to the different states in the process of justification. The Council of Trent assigns the first and most important place to faith, which is styled "the beginning, foundation and root of all justification" (Trent, l.c., cap.viii). Cardinal Pallavicini* (Hist. Conc. Trid., VIII, iv, 18) tells us that all the bishops present at the council fully realized how important it was to explain St. Paul's saying that man is justified through faith. Comparing Bible and Tradition (emphasis added) they could not experience any serious difficulty in showing that fiduciary faith was an absolutely new invention and that the faith of justification was identical with a firm belief in the truths and promises of Divine revelation (l. c.: "illumque [Deum] tanquam omnis justitiae fontem diligere incipiunt"). The next step is a genuine sorrow for all sin with the resolution to begin a new life by receiving holy baptism and by observing the commandments of God. The process of justification(emphasis added) is then brought to a close by the baptism of water, inasmuch as by the grace of this sacrament the catechumen is freed from sin (original and personal) and its punishments, and is made a child of God. The same process of justification is repeated in those who by mortal sin have lost their baptismal innocence; with this modification, however, that the Sacrament of Penance replaces baptism. Considering merely the psychological analysis of the conversion of sinners, as given by the council, it is at once evident that faith alone, whether fiduciary or dogmatic, cannot justify man (emphasis added)
From what has been said it is obvious that justification as understood by Protestants, presents the following qualities: its absolute certainty (certitudo), its equality in all (aequalitas), and finally the impossibility of ever losing it (inamissibilitas). For if it be essential to fiduciary faith that it infallibly assures the sinner of his own justification, it cannot mean anything but a firm conviction of the actual possession of grace.(emphasis added) If, moreover, the sinner be justified, not by an interior righteousness capable of increase or decrease, but through God's sanctity eternally the same, it is evident that all the just from the common mortal to the Apostles and the Blessed Virgin Mary possess one and the same degree of righteousness and sanctity. Finally if, as Luther maintains, only the loss of faith (according to Calvin, not even that) can deprive us of justification, it follows that justification once obtained can never be lost.http://www.newadvent.org/cathen/08573a.htm
See 1Jn.5: 10 He that believeth on the Son of God hath the witness in himself:
In considering the effects of justification it will be useful to compare the Catholic doctrine of real forgiveness of sin with the Protestant theory that sin is merely "covered" and not imputed.
Also, this is an incorrect view of the Protestant view of sin.
That is an Old Testament way of handling sin, covering by the Blood until the final Blood Atonement (Heb.10). Now that the final payment has been made, the Christian's sins are forever removed and when he sins by going back to his Old Nature (Rom7), he can confess those sins and receive full cleansing since they have already been forgiven (1Jn.1:9)
Exactly. We ARE going to sin. But thank God that we can repent, turn away from our sin, and confessit to God so He can cleanse us from all unrighteousness.